1. Field of the Invention
The present invention relates to a method of protection against the unauthorized unscrambling of scrambled television broadcasts, as well as to a device for the implementation of such a method.
The unauthorized unscrambling of pay television broadcasts, commonly known as piracy, is done:
either through the crypto-analysis of the data transmitted, to enable the recovery of the information elements by which the image can be unscrambled or, again, to enable the creation of authorization that has not really been transmitted,
or by the physical analysis of the video signal which will enable the reconstruction of the original structure of the image through the use of its content and on the basis of knowledge of the particular features of the scrambling system.
2. Description of the Prior Art
In so-called first-generation systems, it has often been possible, by a simple processing of the video signal, to unscramble the image (scrambled by systems such as the video inversion or synchronization signal elimination systems). In the case of a so-called second generation system, such as those using the "line cut and rotate system" (namely the inversion of the image on at least one randomly determined point at which the line is cut, hereinafter called a "cut point"), it is known that the chief known method of piratical attack lies in the setting up of a horizontal correlation in order to determine the position of the cut point. The vertical correlation cannot, by itself, enable the structure of the image to be re-established and, at any rate, calls for very powerful and fast, hence very costly, computation and storage means.
As a consequence, to obtain a pirate decoder at a competitive price, it appears to be clear that the most frequent piratical attacks made are at the level of the detection of the cut point, either directly by logical analysis or by horizontal correlation or, finally, by both these methods.